The present invention relates to a method of modifying molecular sieves by means of solid state ion exchange and the modified molecular sieves produced thereby. In a more detailed aspect, the present invention relates to hydrogenation catalysts selective as to the form of product produced and a method for hydrogenating olefinic compounds to form alkanes using said catalysts.
A known method for introducing noble metals such as platinum or palladium into medium-pored and wide-pored zeolites is by way of ion exchange in aqueous suspension with the appropriate tetraammine complexes (e.g., Pt(NH.sub.3).sub.4 Cl.sub.2 or Pd(NH.sub.3).sub.4 Cl.sub.2). This method yields a high distribution of the noble metal in the crystallite interior of wide-pored zeolites (e.g., Y) and medium-pored zeolites (e.g., ZSM-5).
However, small-pored zeolites (e.g., ZSM-58, zeolite A) cannot be doped in this manner with platinum or palladium because the noble-metal complex is too bulky to be able to diffuse into the eight-membered ring channels of these zeolites. The only known method for introducing noble metals into the interstices of small-pored zeolites is to add the noble-metal complex in the desired amount to the synthesis gel for the zeolite production prior to crystallization so that the zeolite crystallizes around the complex (P. B. Weisz and V. J. Frilette, J. Phys. Chem. (1960), volume 64, page 382; P. B. Weisz et al., J. Catal. (1962), volume 1, pages 307-312). A shape-selective hydrogennation catalyst can be produced in a known manner by introducing for example Pt(NH.sub.3).sub.4 Cl.sub.2 into the synthesis gel for zeolite A.
The known method for doping small-pored zeolites has the disadvantage that it is not universally applicable to all zeolite types. The presence of slight amounts of impurity, as in this instance the noble-metal complex, in the synthesis gel hinders the crystallization of the desired zeolite.